It splits the globe into two -- so when it's Tuesday morning on the west side of the line, it's still Monday morning on the east side of it.

Until last year, Samoa and American Samoa celebrated the new year on the same day. But then Samoa hopped west of the line so it could trade more easily with countries such as Australia and New Zealand.

Because the date line is not fixed by any international law or agreement, it can zig and zag to accommodate such government and business interests.